In the fabrication of hot-wire angular rate sensors (ARS) of the type described in U.S. Pat. No. 4,020,700 to Lopiccolo et al, a critical step involves the bonding of the pair of temperature sensitive resistance wires to their mounting posts in the ARS sensor plug assembly. The wires are tungsten, and are less than filament size with a wire diameter on the order of 0.000200 inches (0.00051 cm). The mounting posts are typically KOVAR.RTM. with a diameter on the order 0.020 inches (0.051 cm). The wires must be bonded in electrical contact to the exposed upper surface of the posts.
In the operation of the ARS each resistance wire is an active element in one leg of a balanced bridge. The wire mounted plug assembly is located at one end of a jet chamber housed in a fluid filled casing, and the wires are cooled differentially by a fluid jet in dependence on Coriolis deflection of the jet during angular rotation of the ARS. The resulting bridge imbalance produces a differential voltage signal whose magnitude is proportional to the angular velocity of the sensor. The absence of rotation (sensor null state) results in equal cooling of the wires and the differential output signal from the bridge is ideally zero.
The angular rate is expressed in volts/degree/second. In operation the ARS is used with a servo control system to reduce the angular rate to zero. The voltage output should be zero at this point but in practice there is always a small residual voltage which has to be calibrated for each individual sensor. The main object of the invention is to reduce this residual null voltage (null error) to the smallest possible amount as described below. This null error is critically dependent on the existence of balanced thermal conductivity through the sensor wires and mounting posts, and on the balanced geometric positioning of the wires from the jet stream centerline. In operation, the wires achieve operating temperatures approximately 200.degree. F. above ambient. Heat transfer from the wires is primarily through convection cooling by the jet (approximately 80%), with 12-15% of the heat transfer provided by conduction through the mounting posts, and the balance through radiation. Relative offsets in positioning the wires from the jet centerline will cause unequal convective cooling. Imbalanced thermal conductivity (wire-to-post) will cause unequal conductive cooling. Either of these conditions result in null error.
To provide balanced conductivity the wires are resistance matched to achieve near zero differences. The mounting posts, due to their 100.times. larger diameter than the wires, are effectively balanced. However, thermal conductivity of the different wire-to-post bonds may not be balanced due to variations in bond integrity, inherent in the joining of fine diameter wires to the much larger surface area of the posts and to unequal effective wire lengths due to imprecise location of the weld spots. In the bonding process the wire cannot be directly heated since the wire will melt before the post surface with its relatively large thermal mass.
The prior art bonding techniques include the use of resistance brazing. The positive and negative electrodes of the resistance brazer are placed astride the wire in contact with the post mounting surface. Current flow between the electrodes melts the contact surface area beneath the wire. The melted post material acts as a braze filler metal to fillet the wire and form a joint; after which the electrodes are removed. The process is repeated for the opposite end of the wire post connection.
This type brazing process has a number of disadvantages. Since the resistance braze requires good electrical contact to the post surface, impurities on the contact surface affect the degree of melting. In some cases this may lead to a "cold braze" which, in addition to providing a poor wire-to-post mechanical bond, provides a poor electrical connection. This affects thermal conductivity to the post.
In addition, the contact pressure occurring with placement of the brazer electrodes on the post create bending of the post. When the brazer is removed the post springs back. The spring back may deform the wire. If the wire is attached at one end, spring back of the second end may place the wire in tension, resulting in wire breakage. In either case the wire position relative to the jet centerline may be altered. Since balanced positioning of the wires require geometric position accuracies of 0.0005 inches, or better, this alone provides major performance degradation.
Another disadvantage of the resistance brazing process is that it requires the post to be of a relatively high resistivity material which significantly restricts post material choices. Substantial performance improvements can be realized if greater latitude in post material selection is available.